In accordance with the recent trend toward miniaturization and lightness of portable electronic devices, the necessity for high performance and large capacity of a battery used as a power source of these devices has been increased.
A battery may generate power using materials capable of being electrochemically reacted in a cathode and an anode. As a representative example of the battery as described above, there is a lithium secondary battery generating electrical energy by a change in chemical potential when lithium ions are intercalated into/deintercalated from the cathode and the anode.
The lithium secondary battery is manufactured by using materials capable of reversibly intercalating and deintercalating lithium ions as a cathode active material and an anode active material, and filling an organic electrolyte or a polymer electrolyte between the cathode and the anode.
As the cathode active material of the lithium secondary battery, a lithium complex metal compound has been used. As examples of the lithium complex metal compounds, complex metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiMnO2, and the like, have been studied.
Among the cathode active materials, Mn based cathode active materials such as LiMn2O4, LiMnO2, and the like, are attractive materials having advantages in that they are easily synthesized, are relatively cheap, have the most excellent thermal stability at the time of over-charge as compared to other active materials, and cause little environmental contamination, but have a disadvantage in that capacity is small.
Since LiCoO2 has excellent electric conductivity, a high battery voltage of about 3.7V or so, and excellent cycle life characteristics, stability, and discharge capacity, LiCoO2 is a representative cathode active material commercialized and being sold in the market. However, since LiCoO2 is expensive and occupies 30% of a cost of the battery, there is a problem in that price competitiveness is deteriorated.
Further, among the above-mentioned cathode active materials, LiNiO2 has the highest discharge capacity in view of battery characteristics but has a disadvantage in that it is difficult to synthesize LiNiO2. Further, a high oxidation state of nickel may deteriorate cycle life of a battery and electrodes, cause excessive self-discharge, and deteriorate reversibility. In addition, it is not easy to completely secure stability, such that it is difficult to commercialize LiNiO2.
As the related art, a method of coating a phosphorus compound to impart ion conductivity or roles of a protective layer against metal elusion and side reactions in order to improve performance of a cathode active material has been disclosed below.
A cathode active material for a lithium secondary battery, having a surface on which Li3PO4 is coated for safety and high capacity of a battery has been disclosed in KR1169947. However, in a method of physically dry-coating Li3PO4, which is a coating material, it is impossible to improve the structure in an utmost surface of the cathode active material and a chemical reaction with remaining Li does not occur.
Further, a cathode active material of which high-rate capability and cycle characteristics are improved by containing an oxide coating layer formed on a core of the cathode active material has been disclosed in KR2009-0077163. However, in a method of pre-preparing a metal phosphate and coating the pre-prepared metal phosphate on an active material, since binding strength of the pre-prepared metal phosphate is high, at the time of coating, a reaction with a cathode material is not sufficiently carried out, such that there is a limitation in improving a surface structure, and only an effect of coating a single oxide phosphate has been disclosed in KR2009-0077163.
In addition, a technology of decreasing interface resistance using a transition metal reduction layer self-formed by reactions between a transition metal and a compound having a polyanion structure on a surface of a cathode active material contacting a reaction suppression part has been disclosed in JP2011-233246. However, this technology is a technology applied to an all-solid-state battery, and in the case of an organic solvent electrolyte system except for the all-solid-state battery, an effect of the technology was not stated, and an effect at a high voltage in accordance with the recent trend toward high voltage was also not stated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.